Synthesis Of Monodimentional Medium-Pore Molecular Sieves And Their Effects On Hydroisomerization Of N-Alkane

The removal of long chain normal paraffins from diesel and lubricating oils is essential to produce oils with acceptable cold flow property. Long-chain n-alkane hydroisomerization catalysts can isomerize normal paraffins to isoparaffins which are efficient parts in the oils. Therefore, it is significant to develop high quality long-chain n-alkane hydroisomerization catalysts for excellent properties of diesel or lubricating oils and more oils yield. Bifunctional catalysts with mild strength acid, monodimentional medium-pore molecular sieve as acid function and Pt or Pd as metal function are generally found to be high isomers selective. Otherwise the monodimentional medium-pore molecular sieves are of different types and properties, and n-alkane hydroisomerization porformances of monodimentional medium-pore molecular sieves were seldom researched. Thus it is of actual benefit to research synthesis of monodimentional medium-pore molecular sieves and their effects on n-alkane hydroisomerization porformances.In this paper, four typical monodimentional medium-pore molecular sieves SAPO-41, SAPO-11, ZSM-12 and ZSM-22 were synthesized with hydrothermal method and characterized by XRD, SEM, N2 adsorption desorption, XRF, MAS NMR, TG for crystalline phase, crystallinity, morphology, pore properties, elemental composition, atom environment, embraced materials. The samples of SAPO-41, SAPO-11, ZSM-12 and ZSM-22 obtained were highly crystallized and thermally stablized. The influencing factors of SAPO-41 and ZSM-12 syntheses were sysmetically researched, and the condition regions of pure SAPO-41 and ZSM-12 were determined. Gradual Heating was used to synthesize SAPO-41, the crystallization time was reduced from 50h to 35h, and more Si was induced resulting in more acid sites with this method. When the pseudo-boehmite with higher pore volume was used, a large amount of SAPO-11 was occured and then thansformed to SAPO-41 completely. Al-rich ZSM-12 was synthesized in order to reduce its acid strength.The pore channel structute and acidity of SAPO-41, SAPO-11, ZSM-12 and ZSM-22 were discussed. The hydroisomerization performances of n-dodecane over Pt/monodimentional medium-pore molecular sieves were tested. The catalytic performances indicate that the activities of n-dodecane over Pt/ZSM-12 and Pt/ZSM-22 are higher than that over Pt/SAPO-11 and Pt/SAPO-41, in the meantime Pt/SAPO-11, Pt/SAPO-41 and Pt/ZSM-22 show higher isomers selectivity which reaches 70⁸0% under 85%-90% n-C₁₂ conversion. The selectivity of objective products namely end-methyl-C₁₁ over Pt/SAPO-11, Pt/SAPO-41 and Pt/ZSM-22 is higher than that over Pt/ZSM-12, and only Pt/ZSM-12 produces ethyl-C₁₀ which is usually occurred over large-pore molecular sieves. The cracked products are symmetrical with carben number over Pt/SAPO-11, Pt/SAPO-41 and Pt/ZSM-22, and they are asymmetrical over Pt/ZSM-12 due to secondary cracking.The n-alkane hydroisomerization mechanisms and the effects of acdity and pore channel structure on n-dodecane hydroisomerization were discussed. The conversion activity of n-dodecane is dependent on the acidity of molecular sieves, especially at low temperature. And the selectivity to isomers in hydroisomerization of n-dodecane is highly influenced by the pore channel structure of molecular sieves. The contents of methyl isomers over 12-ring Pt/ZSM-12 are close to that of PCP hypothesis, and 10-ring Pt/SAPO-11, Pt/SAPO-41 and Pt/ZSM-22 produce more end-methyl-C11.The reaction scheme of n-alkane hydroisomerization over monodimentional medium-pore molecular sieves loaded with Pt was established according to the catalytic results and n-alkane carboncation mechanism, and the kinetic equations were provided. The intrinsic rate constant and physisorption equilibrium contant are determined by the acidity or pore channel structure, respectively, and then lead to the diffences of n-alkane hydroisomerization rate.Mesoporous SAPO-11 was synthesized hydrothermally by using carbon black and di-n-propylamime (DPA) as templates for the expected mesopores and micropores, respectively. Mesoporous SAPO-11 was highly crystallized and the mesopore diameters are in range of 10nm-30nm. Mesoporous SAPO-11 has the same acidity with conventional one. The mesoporous SAPO-11 exhibited higher isomers selectivity (92.6%) and yield (83.1%) compared with conventional one at low reaction temperature 563K. The performance of n-dodecane hydroisomerization over Pt/mesoporous SAPO-11 was examined under industrial conditions. And the long term property was also evaluated. The results indicate that Pt/mesoporous SAPO-11 showed good reaction activity (36.7%) and isomers slectivity (95%). And that catalyst shows good stability under industrial reaction pressure and H₂/n-C₁₂ ratio.